Methods for pre-sharpening impregnated cutting structures for bits, resulting cutting structures and drill bits so equipped

ABSTRACT

Processes for pre-sharpening cutting structures comprising particles of superabrasive material such as diamond grit, dispersed in a metal matrix material such as cemented tungsten carbide. Matrix material may be removed from a surface of a cutting structure to a desired depth to expose superabrasive particles within the matrix material adjacent the surface, and to increase exposure of partially exposed superabrasive particles at the surface. Electrodischarge machining (EDM), laser machining, electrolytic etching and chemical etching may also be employed. Pre-sharpened cutting structures are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/779,307, filed Feb. 27, 2013, pending, the disclosure of which ishereby incorporated herein in its entirety by this reference.

FIELD

The present disclosure relates generally to methods for pre-sharpeningso-called “impregnated” cutting structures comprising superabrasivematerial particles fixed in a matrix of metal material, such cuttingstructures being secured to or formed integrally with a body of a rotarydrill bit, as well as to resulting cutting structures and drill bits soequipped.

BACKGROUND

Impregnated drag bits are used conventionally for drilling hard and/orabrasive rock formations, such as sandstones. These impregnated drillbits typically employ a cutting face composed of superabrasive cuttingparticles, such as natural or synthetic diamond grit, dispersed within amatrix of wear-resistant material. As such a bit drills, the matrix andembedded diamond particles wear, worn cutting particles are lost and newcutting particles are exposed. These diamond particles may either benatural or synthetic and may be cast integral with the body of the bit,as in low-pressure infiltration, or may be preformed separately, as inhot isostatic pressure infiltration, and attached to the bit by brazingor furnaced to the bit body during manufacturing thereof by aninfiltration process.

Conventional impregnated bits generally exhibited a poor hydraulicsdesign by employing a crow's foot to distribute drilling fluid acrossthe bit face and providing only minimal flow area. Further, conventionalimpregnated bits did not drill effectively when the bit encounteredsofter and less abrasive layers of rock, such as shales. When drillingthrough shale, or other soft formations, with a conventional impregnateddrag bit, the cutting structure tended to quickly clog or “ball up” withformation material, making the drill bit ineffective. The softerformations can also plug up fluid courses formed in the drill bit,causing heat buildup and premature wear of the bit. Therefore, whenshale-type formations were encountered, a more aggressive bit wasdesired to achieve a higher rate-of-penetration (ROP). It followed,therefore, that selection of a bit for use in a particular drillingoperation became more complicated when it was expected that formationsof more than one type would be encountered during the drillingoperation.

Moreover, during the drilling of a well bore, the well may be drilled inmultiple sections wherein at least one section is drilled followed bythe cementing of a tubular metal casing within the borehole. In someinstances, several sections of the well bore may include casing ofsuccessively smaller sizes, or a liner may be set in addition to thecasing. In cementing the casing (such term including a liner) within theborehole, cement is conventionally disposed within an annulus definedbetween the casing and the borehole wall by flowing the cementdownwardly through the casing to the bottom thereof and then displacingthe cement through a so-called “float shoe” such that it flows backupwardly through the annulus. Such a process conventionally results in amass or section of hardened cement proximate the float shoe and formedat the lower extremity of the casing. Thus, in order to drill the wellbore to further depths, it becomes necessary to first drill through thefloat shoe and mass of cement.

Conventionally, the drill bit used to drill out the cement and floatshoe did not exhibit the desired design for drilling the subterraneanformation, which lies therebeyond. Those drilling the well bore werethen often faced with the decision of changing out drill bits after thecement and float shoe had been penetrated or, alternatively, continuingwith a drill bit that may not have been optimized for drilling thesubterranean formation below the casing.

Thus, it was recognized as beneficial to design a drill bit that wouldperform more aggressively in softer, less abrasive formations while alsoproviding adequate ROP in harder, more abrasive formations and fordrilling such formations interbedded with soft and nonabrasive layerswithout requiring increased weight-on-bit (WOB) during the drillingprocess.

Additionally, it was also recognized as advantageous to provide a drillbit with “drill out” features to enable the drill bit to drill through acement shoe and continue drilling the subsequently encounteredsubterranean formation in an efficient manner.

To address these needs, inventors of the assignee of the presentdisclosure developed and implemented a number of superior bit designsoffered as HEDGEHOG® impregnated bits. A variety of structures for suchbits and specific features thereof are disclosed and claimed in U.S.Pat. Nos. 6,510,906; 6,843,333; 7,278,499; 7,497,280; 7,730,976;8,191,657; 8,220,567 and 8,333,814 and in U.S. Patent Publications2010/0122848; 2010/0219000; and 2011/0061943, among others. Thedisclosure of each of the foregoing patents and patent publications ishereby incorporated herein in its entirety by this reference.

Notable features of the HEDGEHOG® impregnated bits include the use ofimpregnated cutting structures protruding above the bit face to anexposure far greater than was previously conventional and formed asposts, the use of nozzles and of relatively deep and wide fluid passagesand junk slots for improved bit hydraulics, the use of polycrystallinediamond compact cutting elements in the bit cone for superiorperformance in interbedded and shaley formations, as well as the use ofthermally stable polycrystalline diamond cutting elements in combinationwith impregnated posts and other impregnated cutting structures forenhanced drill out capability.

However, even such bits conventionally require a “break-in” periodbefore attaining optimum performance, since the superabrasive particlesin the as-formed cutting structures are substantially embedded in thematrix material of the cutting structure. Thus, in operation, aconventional impregnated bit would be run into a well and “broken-in” or“sharpened” by drilling into an abrasive formation at a selected WOB asthe bit is rotated. For the first several feet of penetration, thediamond grit on the ends of the posts or other cutting structuresbecomes more exposed by wear of the relatively softer matrix material,as no substantial volume of diamond is usually exposed on an impregnatedcutting structure as manufactured. As the bit is “sharpened” to enhanceexposure of the diamond grit on formation-engaging surfaces of theimpregnated cutting structures, ROP increases and stabilizes. It hasbeen demonstrated that HEDGEHOG® impregnated bits, once broken in,exhibit an increased ROP over conventional impregnated bits. It haslikewise been shown that HEDGEHOG® impregnated bits exhibit asubstantially similar ROP to that of a conventional impregnated bit butat a reduced WOB.

However, the need to break in impregnated bits to achieve their fullpotential in terms of increased ROP and reduced required WOB isundesirable.

BRIEF SUMMARY

The present disclosure relates to pre-sharpening of impregnated cuttingstructures for use in drilling and enlarging wellbores throughsubterranean formations.

In one embodiment, a method of pre-sharpening a cutting structure forsubterranean use and comprising superabrasive material particlesdispersed in a metal matrix material comprises selecting a surface ofthe cutting structure and removing a depth of the metal matrix materialfrom the selected surface to at least one of enhance exposure ofsuperabrasive particles exposed above the selected surface and exposeportions of unexposed superabrasive particles adjacent the selectedsurface.

In another embodiment, the present disclosure comprises an unusedcutting structure for subterranean use comprising superabrasiveparticles dispersed in a metal matrix material and exhibitingsubstantial exposure of portions thereof above at least one surface ofthe metal matrix material of the cutting structure.

In a further embodiment, the present disclosure comprises a bit forsubterranean use having at least one unused impregnated cuttingstructure thereon, the at least one unused cutting structure comprisingparticles dispersed in a metal matrix material and exhibitingsubstantial exposure of portions thereof above at least one surface ofthe metal matrix material.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 comprises an inverted perspective view of a first embodiment of abit of the present disclosure;

FIG. 2A is a schematic top elevation of portions of a plurality ofblades of the bit of FIG. 1 carrying discrete cutting structures andFIG. 2B is a side sectional elevation taken across line 2B-2B of FIG.2A;

FIG. 3 is an enlarged, inverted perspective view of part of the coneportion of the face of the bit of FIG. 1, showing wear of discrete,diamond grit-impregnated cutting structures and PDC cutters;

FIG. 4 is a top elevation of the bit of FIG. 1 after testing, showingwear of the discrete cutting structures and PDC cutters;

FIG. 5 is a top elevation of a second embodiment of the bit of thepresent disclosure;

FIG. 6 is an inverted perspective view of the bit of FIG. 5;

FIG. 7 is an inverted perspective view of a bit according to anotherembodiment of the present disclosure;

FIG. 8 is an inverted perspective view of a bit according to yet anotherembodiment of the present disclosure;

FIG. 9A is an elevational side view of a cutting structure andassociated discrete protrusion as indicated by section line 9A-9A inFIG. 8;

FIG. 9B is an elevational side view of a cutting structure andassociated discrete protrusion according to another embodiment of thepresent disclosure;

FIG. 9C is an elevational side view of a cutting structure andassociated discrete protrusion according to yet another embodiment ofthe present disclosure;

FIGS. 10A through 10D are photographs, respectively, of an end surfaceof an impregnated cutting structure before pre-sharpening, an endsurface of an impregnated cutting structure after pre-sharpening, anenlarged photographic view of a portion of an end surface afterpre-sharpening, and a comparative perspective photograph of end surfacesof impregnated cutting structures before and after pre-sharpening;

FIG. 11 A is a photograph of an impregnated cutting structure with anelectrode of an EDM machine above it, and FIG. 11B is a photographicside view of an impregnated cutting structure with an electrode of anEDM machine in contact with an end surface of the cutting structure forpre-sharpening;

FIG. 12A is a side elevation of a post-shaped, round impregnated cuttingstructure having a flat end surface to be pre-sharpened with an EDMelectrode having a flat working end proximate the flat end surface;

FIG. 12B is a side elevation of an impregnated cutting structure havinga tapered end to be pre-sharpened with an EDM electrode having asaddled-shaped working end proximate the tapered end; and

FIG. 12C is a side elevation of an impregnated cutting structure havingan arcuate side portion to be pre-sharpened with an EDM electrode havinga concave working end proximate the arcuate side.

DETAILED DESCRIPTION

The process of enhancing exposure of the abrasive particles of cuttingstructures according to embodiments of the present disclosure may bereferred to herein as “pre-sharpening,” which in its broadest sensecomprises removing matrix material from a surface of a cutting structureto increase exposure of abrasive particles dispersed in the matrixmaterial at or near the surface. As used herein, the term “cuttingstructure” means and includes, in its broadest sense, any structure of adrill bit comprising particulate abrasive material dispersed in a matrixmaterial and located for eventual engagement with a subterraneanformation material during a drilling operation. As used herein, the term“bit” means and include any type of bit or tool used for drilling duringthe formation or enlargement of a wellbore in a subterranean formationand includes, for example, fixed-cutter bits, roller cone bits,percussion bits, core bits, eccentric bits, bicenter bits, reamers,mills, drag bits, hybrid bits, and other drilling bits and tools knownin the art.

Referring now to FIGS. 1-3 of the drawings, a first embodiment of a bit10 of the present disclosure is depicted in perspective, bit 10 beinginverted from its normal face-down operating orientation for clarity.Bit 10 is, by way of example only, of 8.5″ diameter and includes amatrix-type bit body 12 having a shank 14 for connection to a drillstring (not shown) extending therefrom opposite bit face 16. A pluralityof (in this instance, twelve (12)) blades 18 extends generally radiallyoutwardly in linear fashion to gage pads 20 defining junk slots 22therebetween.

The discrete, impregnated cutting structures 24 comprise posts extendingupwardly (as shown in FIG. 1) on blades 18 from the bit face 16. Thecutting structures 24 may be formed as an integral part of thematrix-type blades 18 projecting from a matrix-type bit body 12 byhand-packing diamond grit-impregnated matrix material in mold cavitieson the interior of the bit mold defining the locations of the cuttingstructures 24 and blades 18 and, thus, each blade 18 and associatedcutting structure 24 defines a unitary structure. It is noted that thecutting structures 24 may be placed directly on the bit face 16,dispensing with the blades. However, as discussed in more detail below,it may be preferable to have the cutting structures 24 located on theblades 18. It is also noted that, while discussed in terms of beingintegrally formed with the bit 10, the cutting structures 24 may beformed as discrete individual segments, such as by hot isostaticpressing, and subsequently brazed or furnaced onto the bit 10.

Discrete cutting structures 24 are mutually separate from each other topromote drilling fluid flow therearound for enhanced cooling andclearing of formation material removed by the diamond grit. Discretecutting structures 24, as shown in FIG. 1, are generally of a round orcircular transverse cross-section at their substantially flat, outermostends 26, but become more oval with decreasing distance from the face ofthe blades 18 and thus provide wider or more elongated (in the directionof bit rotation) bases 28 (see FIGS. 2A and 2B) for greater strength anddurability. As the discrete cutting structures 24 wear (see FIG. 3), theexposed cross-section of the posts increases, providing progressivelyincreasing contact area for the diamond grit with the formationmaterial. As the discrete cutting structures 24 wear down, the bit 10takes on the configuration of a heavier-set bit more adept atpenetrating harder, more abrasive formations. Even if discrete cuttingstructures 24 wear completely away, the diamond-impregnated blades 18will provide some cutting action, reducing any possibility of “ring-out”and having to pull the bit 10.

While the cutting structures 24 are illustrated as exhibiting posts ofcircular outer ends and oval shaped bases, other geometries are alsocontemplated. For example, the outermost ends 26 of the cuttingstructures 24 may be configured as ovals having a major diameter and aminor diameter. The base 28 adjacent the blade 18 might also be oval,having a major and a minor diameter, wherein the base 28 has a largerminor diameter than the outermost end 26 of the cutting structure 24. Asthe cutting structure 24 wears toward the blade 18, the minor diameterincreases, resulting in a larger surface area. Furthermore, the ends ofthe cutting structures 24 need not be flat, but may employ slopedgeometries. In other words, the cutting structures 24 may changecross-sections at multiple intervals, and tip geometry may be separatefrom the general cross-section of the cutting structure 24. Other shapesor geometries may be configured similarly. It is also noted that thespacing between individual cutting structures 24, as well as themagnitude of the taper from the outermost ends 26 to the blades 18, maybe varied to change the overall aggressiveness of the bit 10 or tochange the rate at which the bit is transformed from a light-set bit toa heavy-set bit during operation. It is further contemplated that one ormore of such cutting structures 24 may be formed to have substantiallyconstant cross-sections, if so desired, depending on the anticipatedapplication of the bit 10.

Discrete cutting structures 24 may comprise a synthetic diamond grit,such as, for example, DSN-47 Synthetic diamond grit, commerciallyavailable from DeBeers of Shannon,

Ireland, which has demonstrated toughness superior to natural diamondgrit. The tungsten carbide matrix material with which the diamond gritis mixed to form discrete cutting structures 24 and supporting blades 18may include a fine grain carbide, such as, for example, DM2001 powdercommercially available from Kennametal Inc., of Latrobe, Pa. Such acarbide powder, when infiltrated, provides increased exposure of thediamond grit particles in comparison to conventional matrix materialsdue to its relatively soft, abradable nature. The base 30 of each blade18 may be formed of, for example, a more durable 121 matrix material,obtained from Firth MPD of Houston, Texas. Use of the more durablematerial in this region helps to prevent ring-out even if all of thediscrete cutting structures 24 are abraded away and the majority of eachblade 18 is worn.

It is noted, however, that other particulate abrasive materials may besuitably substituted for those discussed above. For example, thediscrete cutting structures 24 may include natural diamond grit, or acombination of synthetic and natural diamond grit. Particles of cubicboron nitride may also be employed, in addition to or in lieu of diamondparticles. Alternatively, the cutting structures 24 may includesynthetic diamond pins. Additionally, the particulate abrasive materialmay be coated with a single layer or multiple layers of one or morematerials, as known in the art and disclosed in U.S. Pat. Nos.4,943,488, 5,049,164 and 8,220,567, the disclosure of each of which ishereby incorporated herein in its entirety by reference. Such materialsmay include, for example and without limitation, a refractory metal, arefractory metal carbide, and a refractory metal oxide. In oneembodiment, the coating may exhibit a thickness of approximately 1 to 10microns. In another embodiment, the coating may exhibit a thickness ofapproximately 2 to 6 microns. In yet another embodiment, the coating mayexhibit a thickness of less than 1 micron. In a further embodiment, thecoating may exhibit a thickness of up to about 250 microns.

The choice of grit size may be application specific, and generally asubstantially uniform grit size, categorized by mesh size, may beemployed in a given cutting structure although the disclosure is not solimited. Suitable mesh sizes may include, by way of example, 30/40 (660stones per carat (SPC)), 25/25 (420 SPC), 20/25 (210 SPC) and 18/20 (150SPC). A larger grit size, for example, 150 SPC or 210 SPC may be moresuitable for drilling non-abrasive formations such as shale, while asmaller grit size, for example, 420 SPC or 660 SPC, may be employed in amore demanding, abrasive formation.

In all of the cutting structures 24 described above, an outermost end 26of each cutting structure 24 and, if desired, one or more othersurfaces, may be pre-sharpened after formation thereof to enhanceexposure of particles of the abrasive material above an adjacent surfaceof the matrix material of the cutting structure 24. Such exposure may beenhanced before or after cutting structures 24 are located on a bit bodyif cutting structures 24 are preformed, such as by hot isostaticpressing, and of course, exposure of abrasive particles on cuttingstructures 24 formed integrally with a bit body may be enhanced in situ.Consequently, bit 10 with its pre-sharpened cutting structures 24 isenabled to drill efficiently in terms of applied WOB and resulting ROP,without a break-in period required of conventional impregnated bits. Insome embodiments, exposure of the abrasive particles visible at asurface of the cutting structure 24 may be enhanced, by way of exampleonly, to a height above a surface of adjacent matrix material of betweenabout twenty percent (20%) of a size of the abrasive particles and aboutsixty percent (60%) of a size of the abrasive particles.

Referring now to FIG. 4, the radially innermost ends of two blades 18extend to the centerline of bit 10 and carry cutting elements, shown asPDC cutters 32, in conventional orientations, with cutting facesoriented generally facing the direction of bit rotation. PDC cutters 32are located within a cone portion 34 of the bit face 16. The coneportion 34, best viewed with reference to FIG. 1, is the portion of thebit face 16 wherein the profile is defined as a generally cone-shapedsection about the centerline of intended rotation of the bit 10. Whileboth discrete cutting structures 24 and PDC cutters 32 are carried bythe bit 10, as is apparent in FIGS. 1 and 4, there is desirably agreater quantity of the discrete cutting structures 24 than there arePDC cutters 32.

The PDC cutters may comprise cutters having a PDC jacket or sheathextending contiguously with, and to the rear of, the PDC cutting faceand over the supporting substrate. For example, a cutter of this type isoffered by the assignee of the present invention, as NIAGARA™ cutters.Such cutters are further described in U.S. Pat. No. 6,401,844, issuedJun. 11, 2002, and entitled CUTTER WITH COMPLEX SUPERABRASIVE GEOMETRYAND DRILL BITS SO EQUIPPED. This cutter design provides enhancedabrasion resistance to the hard and/or abrasive formations typicallydrilled by impregnated bits, in combination with enhanced performance(ROP) in softer, nonabrasive formation layers interbedded with such hardformations. It is noted, however, that alternative PDC cutter designsmay be implemented. Rather, PDC cutters 32 may be configured of variousshapes, sizes, or materials as known by those of skill in the art. Also,other types of cutting elements may be formed within the cone portion 34of the bit 10 depending on the anticipated application of the bit 10.For example, the cutting elements formed within the cone portion 34 mayinclude cutters formed of thermally stable diamond product (TSP),natural diamond material, or impregnated diamond.

Again referring to FIG. 4 of the drawings, bit 10 employs a plurality(for example, eight (8)) ports 36 over the bit face 16 to enhance fluidvelocity of drilling fluid flow and better apportion the flow over thebit face 16 and among fluid passages 38 between blades 18 and extendingto junk slots 22. This enhanced fluid velocity and apportionment helpsprevent bit balling in shale formations, for example, which phenomenonis known to significantly retard ROP. Further, in combination with theenhanced diamond exposure of bit 10, the improved hydraulicssubstantially enhances drilling through permeable sandstones.

Referring back to FIG. 1, by way of illustration only, the gage pads ofthe illustrated embodiment may be approximately 3 inches long, eachcomprising approximately 1.5 inches of thermally stable product (TSP)diamond and diamond grit-impregnated matrix, and approximately 1.5inches of carbide bricks and K-type natural diamonds. Such anarrangement may likewise be applied to bits of differing diameters.

Referring now to FIGS. 5 and 6 of the drawings, another embodiment 100of the bit according to the disclosure is depicted. Features previouslydescribed with reference to bit 10 are identified with the samereference numerals on bit 100. It will be noted that there is a largernumber of blades 18 on bit 100 than on bit 10, and that the blades 18carrying cutting structures 24 spiral outwardly from the cone portion 34of bit 100 toward the gage pads 20 (see FIG. 6). The use of the curved,spiraled blades 18 provides increased blade length and thus greaterredundancy of coverage of discrete cutting structures 24 at each radius.It should also be noted that there are a larger number of ports 36 onbit face 16 for fluid distribution typically through nozzles (not shown)installed in the ports 36. The ports 36 within the cone portion 34 arepreferably of larger diameter than those outside of the cone portion 34.Alternatively, the blades 18 may be formed in other shapes or patterns.For example, the blades 18 may be formed to extend outwardly from thecone portion 34 in a serpentine fashion, each blade forming an “S” shapeas it travels across the bit face 16 toward the gage pads 20.

In all of the cutting structures described above, an outermost end 26 ofeach cutting structure 24 and, if desired, one or more other surfaces,may be pre-sharpened after formation thereof to enhance exposure ofparticles of the abrasive material above an adjacent surface of matrixmaterial. Such exposure may be enhanced before or after cuttingstructures 24 are located on a bit body if cutting structures 24 arepreformed, such as by hot isostatic pressing, and of course exposure ofabrasive particles on cutting structures 24 formed integrally with a bitbody may be enhanced in situ. Consequently, bit 100 with itspre-sharpened cutting structures 24 is enabled to drill efficiently interms of applied WOB and resulting ROP, without a break-in periodrequired of conventional impregnated bits. In some embodiments, exposureof the abrasive particles visible at a surface of the cutting structuremay be enhanced, by way of example only, to a height above a surface ofadjacent matrix material of between about twenty percent (20%) of a sizeof the abrasive particles and about sixty percent (60%) of a size of theabrasive particles.

Referring now to FIG. 7, a bit 120 is shown in accordance with anotherembodiment of the present disclosure. As with the embodiments describedabove, the bit 120 includes a matrix-type bit body 12 having a shank 14,for connection with a drill string, extending therefrom opposite a bitface 16. The bit 120 also includes a plurality of blades 18 extendinggenerally radially outwardly to gage pads 20 which define junk slots 22therebetween.

Cutting structures 124 comprising posts extend upwardly from the blades18 and are formed as described hereinabove. The cutting structures 124,as shown in FIG. 7, exhibit generally flat, oval cross-sectionalgeometries that are substantially constant from their outer ends 126down to where they interface with the blades 18. It is noted, however,that the cutting structures 124 may exhibit other cross-sectionalgeometries, including those which change from their outer ends 126 towhere they interface with the blades 18, as previously described herein.

The bit 120 does not necessarily include additional cutters, such as PDCcutters, in the cone portion 34 (FIG. 1) of the bit face 16. Rather, thecone portion 34 may include additional cutting structures 124A therein.The cutting structures 124A located within the cone portion 34 mayexhibit geometries that are similar to those which are more radiallydisposed on the bit face 16, or they may exhibit geometries that aredifferent from those which are more radially disposed on the bit face16. For example, cutting structure 124A, as shown in FIG. 7, whileexhibiting a generally flat, oval outer end 126A, exhibits dimensionswhich are different from those more radially outwardly disposed suchthat the major and minor axes of the generally oval geometry are rotatedapproximately 90° relative to the cutting structure 124B adjacentthereto.

In all of the cutting structures described above with respect to FIG. 7,an outermost end 126 of each cutting structure 124 (such term includingcutting structures designated as 124A and 124B) and, if desired, one ormore other surfaces, may be pre-sharpened after formation thereof toenhance exposure of particles of the abrasive material above an adjacentsurface of matrix material. Such exposure may be enhanced before orafter cutting structures 124 are located on a bit body if cuttingstructures 124 are preformed, such as by hot isostatic pressing, and ofcourse exposure of abrasive particles on cutting structures 124 formedintegrally with a bit body may be enhanced in situ. Consequently, bit120 with its pre-sharpened cutting structures 124 is enabled to drillefficiently in terms of applied WOB and resulting ROP, without abreak-in period required of conventional impregnated bits. In someembodiments, exposure of the abrasive particles visible at a surface ofthe cutting structure may be enhanced, by way of example only, to aheight above a surface of adjacent matrix material of between abouttwenty percent (20%) of a size of the abrasive particles and about sixtypercent (60%) of a size of the abrasive particles.

Referring now to FIG. 8, a drill bit 130 is shown according to yetanother embodiment of the present disclosure. The drill bit 130 isconfigured generally similar to that which is described with respect toFIG. 7, but includes what may be termed “drill out” features whichenable the bit 130 to drill through, for example, a float shoe and massof cement at the bottom of a casing within a well bore.

Discrete protrusions 132, formed of, for example, a TSP material, extendfrom a central portion of the generally flat outer end 126 of some orall of the cutting structures 124. As shown in FIG. 9A, the discreteprotrusions 132 may exhibit a substantially triangular cross-sectionalgeometry having a generally sharp outermost end, as taken normal to theintended direction of bit rotation, with the base of the triangleembedded in the cutting structure 124 and being mechanically andmetallurgically bonded thereto. The TSP material may be coated with, forexample, a refractory material such as that described hereinabove.

The discrete protrusions 132 may exhibit other geometries as well. Forexample, FIG. 9B shows a discrete protrusion 132′ having a generallysquare or rectangular cross-sectional geometry as taken normal to theintended direction of bit rotation and, thus, exhibits a generally flatoutermost end. Another example is shown in FIG. 9C wherein the discreteprotrusion 132″ exhibits a generally rounded or semicircularcross-sectional area as taken normal to the intended direction of bitrotation.

As shown in FIG. 8, the cross-sectional geometry of each of the discreteprotrusions 132, taken substantially parallel with the generally flatouter end 126 of its associated cutting structure 124, is generallycongruous with the cross-sectional geometry of the cutting structure124. It is noted that a portion of each of the cutting structure's outerend 126 surrounding the discrete protrusions 132 remains exposed. Thus,the discrete protrusions 132 do not completely conceal, or otherwisereplace, the generally flat outer ends 126 of the cutting structures124. Rather, discrete protrusions 132 augment the cutting structures 124for the penetration of, for example, a float shoe and associated mass ofcement therebelow or similar structure prior to penetrating theunderlying subterranean formation.

As with the cutting structures described above in regard to otherembodiments, an outermost end 126 of each cutting structure 124 and, ifdesired, one or more other surfaces, may be pre-sharpened afterformation thereof to enhance exposure of particles of the abrasivematerial above an adjacent surface of matrix material. Such exposure maybe enhanced before or after cutting structures 124 are located on a bitbody if cutting structures 124 are preformed, such as by hot isostaticpressing, and of course exposure of abrasive particles on cuttingstructures 124 formed integrally with a bit body may be enhanced insitu. In some embodiments, exposure of the abrasive particles visible ata surface of the cutting structure may be enhanced, by way of exampleonly, to a height above a surface of adjacent matrix material of betweenabout twenty percent (20%) of a size of the abrasive particles and aboutsixty percent (60%) of a size of the abrasive particles. Consequently,once discrete protrusions 132 have augmented penetration of a float shoeand associated mass of cement and drill bit 130 has engaged formationmaterial to drill ahead and extend the wellbore, the pre-sharpened outerends 126 of cutting structures 124 surrounding discrete protrusions 132are immediately effective in terms of WOB required and ROP achievedwithout a break-in period.

In one embodiment, impregnated cutting structures may be pre-sharpenedby employing an electrodischarge machining (EDM) technique, which isalso known as “spark erosion.” In such a technique, an electrode isdisposed against a workpiece and vibrated, while intermittent electricarcs are applied to break down adjacent metal material of the workpieceinto minute particles. During this processing a dielectric coolant ispumped through a channel in the electrode to wash away the minuteparticles. By disposing a face of an electrode of an EDM machineproximate a surface of an impregnated cutting structure comprising, forexample, diamond grit dispersed in a tungsten carbide matrix material, asurface depth of the tungsten carbide matrix material may be removed toexpose diamond grit at or near the surface proximate the electrode face.Depth of matrix material removal may be controlled manually, by aprogram controlling the EDM machine, or by a physical stop engaging theelectrode to limit its travel. A suitable depth of matrix material maybe selected based at least in part on the diamond grit size employed inthe cutting structure, so that excess diamond grit is not removed fromthe cutting structure surface being pre-sharpened.

In one example, a Cammann Model C-106XA Metal Disintegrator availablefrom Cammann Inc. of Birmingham, Ohio, was employed to presharpen endfaces of impregnated, post-shaped, round cutting structures. A #4 powersetting, a vibration setting of 80-100, and a sensitivity setting of 50were employed. The feed rate was manually controlled, and the depth ofmatrix material removal determined by vertical travel of the electrode.FIG. 10A of the drawings shows an end surface of one of the impregnatedpost test specimens prior to pre-sharpening, while FIG. 10B shows a postend surface after pre-sharpening as described above. As can be readilyseen, exposure of the diamond elements is greatly enhanced. It may benoted that a center portion of the end surface in FIG. 10B has not beensharpened, due to the presence of a hole in the electrode working end(for introducing coolant through the channel in the electrode) used inthe tests. FIG. 10C is an enlarged view of an impregnated post endsurface after pre-sharpening, while FIG. 10D shows facing end surfacesof an as-formed impregnated post on the left and a pre-sharpenedimpregnated post on the right. FIG. 11A depicts a post-shaped roundcutting structure as employed in the example with an electrode suspendedabove it. FIG. 11B is a side view of the electrode in contact with theflat end of a post-shaped round cutting structure to be pre-sharpened.

While flat end surfaces of impregnated, post-shaped, round cuttingstructures were sharpened, other surface configurations may be sharpenedwith suitably configured EDM electrodes. For example, while a flat endE_(F) of an impregnated, post-shaped cutting structure CS1 may besharpened with an electrode EL1 employing a flat working end F asschematically depicted in FIG. 12A, a tapered, rounded end surfaceE_(TR) of a cutting structure CS2 may be sharpened with an electrode EL2employing a saddle-shaped working end S as depicted in FIG. 12B.Further, end flat surfaces S_(F) on one or both sides of the tapered,rounded end surface E_(TR) may be separately sharpened using anelectrode EL1 with a flat working end F disposed at an appropriate anglefor contact. In addition, an arcuate side surface S_(A) of impregnated,post-shaped cutting structure such as cutting structure CS1 may besharpened with an electrode EL3 employing an arcuate, and specificallyconcave, working end C as depicted in FIG. 12C. While channels forcoolant are located within these electrodes, as previously described,they are not shown for enhanced drawing clarity. Sharpening of other,simple and complex surfaces may also be effected using electrodes withappropriately shaped working ends and cooling channels and oriented atsuitable angles with respect to surfaces to be pre-sharpened.

It is also contemplated that other techniques may be employed in apre-sharpening process, including without limitation laser machining,electrolytic etching and chemical etching. An approach with any of thesetechniques, as well as with the EDM technique described above, is toemploy a minimum amount of energy, and particularly applied heat, toremove the matrix material without damaging the superabrasive (e.g.,diamond) particles embedded in the matrix. Significant, heat-induceddegradation of diamond in an ambient environment (e.g., not inertoxygen-free or a vacuum) may commence at a temperature of about 750° C.,due to a tendency toward back-graphitization of the diamond augmented bythe presence of any group VIII catalyst metals which may be present inthe matrix material, which conventionally may be cobalt-cementedtungsten carbide.

Further, it is desirable that the sharpening process create a reasonablyuniform pattern of exposed superabrasive particles above the matrixmaterial surface so that substantially all of a pre-sharpened surfacecommences to cut formation material with a similar degree ofaggressivity.

In tests using post-shaped, round impregnated posttest specimenspre-sharpened as noted above, pre-sharpened test specimens demonstratedexcellent rock cutting efficiency from inception of testing on a visualsingle point (VSP) test machine. In fact, surprisingly, thepre-sharpened test specimens were substantially more efficient thancutting efficiency of blunt (unsharpened) test specimens of the sameconstruction even after a substantial period of testing time.

Additional non-limiting example embodiments of the disclosure aredescribed below.

Embodiment 1

A method of pre-sharpening a cutting structure for subterranean use andcomprising superabrasive material particles dispersed in a metal matrixmaterial, the method comprising:

-   -   selecting at least one surface of the cutting structure; and    -   removing a depth of the metal matrix material from the at least        one selected surface to at least one of enhance exposure of        superabrasive particles exposed above the at least one selected        surface and expose portions of unexposed superabrasive particles        adjacent the at least one selected surface.

Embodiment 2

The method of embodiment 1, wherein removing a depth of the metal matrixmaterial comprises electrodischarge machining the matrix material.

Embodiment 3

The method of embodiment 1, wherein removing a depth of the metal matrixmaterial comprises one of laser machining, electrolytic etching andchemical etching of the matrix material.

Embodiment 4

The method of any of embodiments 1 through 3, wherein removing a depthof the metal matrix material comprises removing a substantially uniformdepth of the matrix material from the at least one selected surface.

Embodiment 5

The method of any of embodiments 1 through 4, wherein removing a depthof the matrix material from the at least one selected surface to atleast one of enhance exposure of superabrasive particles exposed abovethe at least one selected surface and expose portions of unexposedsuperbrasive particles adjacent the at least one selected surfacefurther comprises both enhancing exposure of superabrasive particlesexposed above the at least one selected surface and exposing portions ofunexposed superbrasive particles adjacent the at least one selectedsurface.

Embodiment 6

The method of any of embodiments 1 through 5, wherein removing a depthof the metal matrix material from the at least one selected surfacecomprises removing a depth of the matrix material from at least one of aflat surface, an arcuate surface, and a surface comprising at least flatand arcuate portions.

Embodiment 7

The method of any of embodiments 1 through 6, further comprisingpre-sharpening the cutting structure and subsequently securing thepre-sharpened cutting structure to a portion of a bit.

Embodiment 8

The method of embodiments 1 through 6, further comprising securing thecutting structure to a portion of a bit, and pre-sharpening the cuttingstructure thereafter.

Embodiment 9

The method of any of embodiments 1 through 8, wherein the superabrasivematerial particles comprise at least one of natural diamond grit,synthetic diamond grit, and cubic boron nitride.

Embodiment 10

The method of any of embodiments 1 through 9, wherein the superabrasivematerial particles comprise a coating including a refractory material.

Embodiment 11

The method of embodiment 10, wherein the refractory material comprisesat least one of a refractory metal, a refractory metal carbide and arefractory metal oxide.

Embodiment 12

An unused impregnated cutting structure for subterranean use comprisingsuperabrasive material particles dispersed in a metal matrix materialand exhibiting substantial exposure of portions thereof above at leastone surface comprising the metal matrix material.

Embodiment 13

The cutting structure of embodiment 12, wherein the superabrasivematerial particles comprise at least one of natural diamond grit,synthetic diamond grit, and cubic boron nitride.

Embodiment 14

The cutting structure of embodiment 12 or 13, wherein the superabrasivematerial particles comprise a coating including a refractory material.

Embodiment 15

The cutting structure of embodiment 14, wherein the refractory materialcomprises at least one of a refractory metal, a refractory metal carbideand a refractory metal oxide.

Embodiment 16

The cutting structure of any of embodiments 12 through 15, wherein thesuperabrasive material particles are metallurgically bonded to the metalmatrix material through a refractory metal.

Embodiment 17

A bit for subterranean use having at least one unused cutting structurethereon, the at least one unused cutting structure comprisingsuperabrasive particles dispersed in a metal matrix material andexhibiting substantial exposure of portions thereof above at least onesurface of the metal matrix material.

Embodiment 18

The bit of embodiment 17, wherein the superabrasive material particlescomprise at least one of natural diamond grit, synthetic diamond grit,and cubic boron nitride.

Embodiment 19

The bit of embodiment 17 or 18, wherein the superabrasive materialparticles comprise a coating including a refractory material.

Embodiment 20

The bit of embodiment 19, wherein the refractory material comprises atleast one of a refractory metal, a refractory metal carbide and arefractory metal oxide.

Embodiment 21

The bit of any of embodiments 17 through 20, wherein the superabrasivematerial particles are metallurgically bonded to the metal matrixmaterial through a refractory metal material.

While the present disclosure been described with reference to certainillustrated embodiments, those of ordinary skill in the art willrecognize and appreciate that it is not so limited. Additions, deletionsand modifications to the embodiments illustrated and described hereinmay be made without departing from the scope of the disclosure asdefined by the claims herein, and legal equivalents. Similarly, featuresfrom one embodiment may be combined with those of another.

What is claimed is:
 1. A method of pre-sharpening an impregnated cutting structure for subterranean use, the method comprising: providing the impregnated cutting structure including superabrasive material particles dispersed in a matrix material; and removing a depth of the matrix material of at least one formation-engaging surface to at least one of enhance exposure of exposed superabrasive particles to an increased height above the matrix material or expose portions of unexposed superabrasive particles adjacent the matrix material.
 2. The method of claim 1, wherein removing the depth of the matrix material further comprises removing a substantially uniform depth of the matrix material while minimizing damage to the superabrasive material particles to both enhance exposure of the exposed superabrasive particles to the increased height above the matrix material and expose portions of the unexposed superabrasive particles adjacent the matrix material.
 3. The method of claim 1, wherein removing the depth of the matrix material further comprises at least one of machining, electrolytic etching or chemical etching the matrix material.
 4. The method of claim 3, wherein removing the depth of the matrix material comprises removing the depth of matrix material by machining, and machining comprises at least one of electrodischarge machining or laser machining the matrix material.
 5. The method of claim 1, wherein removing the depth of the matrix material includes enhancing aggressiveness of the at least one formation-engaging surface of the impregnated cutting structure prior to use.
 6. The method of claim 1, wherein providing the impregnated cutting structure including the superabrasive material particles dispersed in the matrix material further comprises bonding the superabrasive material particles to the matrix material through at least one refractory material coating applied to the superabrasive material particles.
 7. A pre-sharpened impregnated cutting structure for subterranean use, comprising: a plurality of superabrasive material particles dispersed in a matrix material; at least one formation-engaging surface located on the pre-sharpened impregnated cutting structure; and at least a portion of the superabrasive material particles exhibiting substantial exposure above the at least one formation-engaging surface resulting from removing a depth of matrix material prior to use.
 8. The cutting structure of claim 7, wherein the plurality of superabrasive material particles comprises at least one of natural, synthetic or a combination of natural and synthetic superabrasive material particles.
 9. The cutting structure of claim 7, wherein: the depth of matrix material removed prior to use includes a substantially uniform depth of matrix material; and the substantially uniform depth of matrix material is dependent on a grit size of the plurality of superabrasive material particles.
 10. The cutting structure of claim 7, wherein the matrix material comprises a metal matrix material comprising at least one of a tungsten carbide or a cobalt-cemented tungsten carbide.
 11. The cutting structure of claim 10, wherein the plurality of superabrasive material particles is bonded to the metal matrix material through at least one coating applied to the plurality of superabrasive material particles, the at least one coating comprising at least one of a refractory metal, a refractory metal carbide or a refractory metal oxide.
 12. The cutting structure of claim 11, wherein: the plurality of superabrasive material particles is spaced substantially uniform within and mutually separated by the metal matrix material, and the at least one coating is configured to enable the plurality of superabrasive material particles to exhibit substantial exposure above the at least one formation-engaging surface.
 13. The cutting structure of claim 7, wherein the pre-sharpened impregnated cutting structure comprises a cylindrical shape having at least one of a flat end surface, a tapered end surface or an arcuate end surface.
 14. The cutting structure of claim 7, further comprising at least one discrete protrusion extending from an outer end surface of the pre-sharpened impregnated cutting structure, wherein the at least one discrete protrusion is coated with a refractory material and is bonded to the matrix material of the pre-sharpened impregnated cutting structure.
 15. A bit for subterranean use, comprising: a plurality of pre-sharpened impregnated cutting structures comprising: a matrix material; at least one formation-engaging surface located on the plurality of pre-sharpened impregnated cutting structures; and superabrasive material particles dispersed in the matrix material and exhibiting substantial exposure above the at least one formation-engaging surface resulting from removing a depth of the matrix material prior to use.
 16. The bit of claim 15, wherein the plurality of pre-sharpened impregnated cutting structures is configured for at least one of drilling or enlarging wellbores through subterranean formations.
 17. The bit of claim 15, wherein the matrix material further comprises a wear-resistant metal matrix material, the superabrasive material particles being metallurgically bonded to the wear-resistant metal matrix material through a coating.
 18. The bit of claim 15, wherein the plurality of pre-sharpened impregnated cutting structures comprises at least one of sharpened prior to being secured to the bit or sharpened after being integrally formed on the bit prior to using the bit in a wellbore.
 19. The bit of claim 15, wherein at least some pre-sharpened impregnated cutting structures of the plurality of pre-sharpened impregnated cutting structures are discrete and are configured and located to provide a progressively increasing contact area between the superabrasive material particles and a formation.
 20. The bit of claim 19, wherein the plurality of discrete pre-sharpened impregnated cutting structures further comprises protrusions extending from the at least one formation-engaging surface, the protrusions having at least one of a triangular, square, rectangular or rounded cross-sectional geometry. 